Emission computed tomography measurements, in particular positron emission tomography (PET), are valuable methods in particular for obtaining information about metabolic, pharmacological or molecular processes. Since in an emission computed tomography procedure high doses of radiation are to be avoided, narrow limits are set for a maximum radiation dose to be used. Hence measurements using emission computed tomography procedures regularly require measuring times lasting several minutes. If moving objects are examined, with motion of the internal organs being unavoidable particularly when examining living beings, only blurred measurements can be achieved in this way.
A known remedy for this is to correct the motions. Thus it is for example possible to estimate motion data from PET data, in which for example the center of mass of the radiating region is tracked. Alternatively, gating methods are used, i.e. methods in which only detection events of a particular motion phase are used. Gating is possible for example by recording an electrocardiogram during the emission computed tomography measurement and sorting the data on the basis of the cardiac cycle. Besides using data from just one phase, it is also possible to use data from several phases thanks to a motion correction. It should be noted here however that a motion correction from emission tomography data alone is made more difficult by the noisy signal.
Methods are also known in which emission tomography data is corrected using computed tomography data from a trans-irradiating method. This has the advantage that information about the attenuation of the radiation by the tissue can be determined simultaneously using relatively high-contrast images from the computed tomograph systems. However, if a periodic or pseudoperiodic motion of an internal organ is to be corrected, several recordings must be made for each motion cycle. This results in a considerable increase in exposure to radiation.
From C. Tsoumpas et al., Phys. Med. Biol. 56 (2011) 6597-6613, the entire contents of which are hereby incorporated herein by reference, it is known that when using combined PET-MR data, a motion correction of the PET data is also possible using the magnetic resonance data. While the publication exclusively works with simulated PET data, it shows that a motion correction of emission computed tomography data using magnetic resonance data is at least possible. In this case, although the MR measuring time is reduced by optimizing the measurement protocol and adjusting the resolution, only a temporal resolution of 0.7 seconds is achieved. If it is desired to correct faster motions, such as a motion of the heart on the basis of the pulse for example, this temporal resolution is not sufficient. To be able to map a motion at all reliably, more than two images per motion cycle should be recorded. To identify a more precise form of motion or asymmetries of motion, even higher temporal resolutions are required. Simply reducing the measurement resolution may indeed cut the measuring time, but the spatial resolution then deteriorates.